A is a notation of bearing set dimensions, there are a few different sizes A B C etc.
the 51 GMC, in fact any GMC, did not come with a 235, the GMC got a special line of high pressure oiling engines. you cant install a 235 where there was a 228/248/270 and vice versa, the GMC engines are about 5 inches longer which is why the radiator is tucked inside the core support.
the 248 is correct for 1951 GMC. if you have a 248 and 8 leaf springs, I would bet it is a 150 which is a 3/4 ton.
